Enable creation of static userspace probes in incremental builds.

To define USDT probes, dtrace -G makes use of relocations for undefined
symbols: the target address is overwritten with NOPs and the location is
recorded in the DOF section of the output object file. To avoid link
errors, the original relocation is destroyed. However, this means that
the same input object file cannot be processed multiple times, as
happens during incremental rebuilds. Instead, only set the relocation
type to NONE, so that all information required to reconstruct USDT
probes is preserved.

Reported by: bdrewery
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation